In an attempt to define and delineate the human "critical period" we have been longitudinally tracking the monocular visual resolution capabilities (acuity) in infants with esotropia and catarats before and during therapeutic management. Our data indicate that amblyopia begins at 3 months of age for the congenitally esotropic babies. Prior to any form of therapy the deviating eye shows an arrested development but a seemingly normal development for the straight eye. Short periods (e.g., one week) of occluding the straight eye resulted in an acuity trade-off between the two eyes, i.e., an acuity reduction or arrest in the patched eye and an acuity improvement in the deviating eye. Subsequent removal of the patch resulted in a rapid reversion to the previous condition, i.e., the acuity in the previously patched eye increased to above normal levels with a concomitant reduction in acuity of the deviating eye to the initially low amblyopic level. Infant undergoing less severe patching regimens (e.g., one hour/day or two hours/day) display similar acuity trade-offs between the eyes. Tha acuity fluctuations in these cases, however, take longer periods of time to show effects as dramatic as constant occlusion. In infants with bilateral congenital cataracts, three weeks of bilateral visual deprivation from birth produces no deleterious effects on visual acuity. The interoperative period of unbalanced visual input (i.e., left eye with cataract and right eye aphakic without correction) had no effect on the initial acuity of the later operated eye. Binocular deprivation due to completion of cataracts, from 3 to 6 months of life, arrests the development of visual acuity which is recoverable in due course of time with full optical correction. Post-operative acuities of the optically corrected aphakic eyes show a normal developmental course. In one unilateral congenital cataract baby, the first five months of monocular deprivation (due to the cataract) arrested normal acuity development. However, with only adequate optical correction (and no occlusion of the normal eye), the acuity of the previously cataractous eye recovered and developed normally.